I'm trying write a C program that sends an UDP packet to a given IP adress and waits for an ICMP response of a router telling that the time to live expired. It's kept very simple because I just want to understand the mechanism at first. What I need is someone checking my code giving feedback on what's wrong and what's missing. I'm very unexperienced at C-programming but I have to do it as an assignment - giving my best to understand it...
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <netinet/ip.h>
// The packet length
#define PCKT_LEN 8192
// Source IP, source port, target IP, target port from the command line arguments
int main(int argc, char *argv[])
{
int send, recv, resp;
int ttl = 1; // Time to live
char buffer[PCKT_LEN];
// Destination address
struct sockaddr_in dst;
// ICMP Response
struct msghdr icmp;
memset(buffer, 0, PCKT_LEN);
// Create a raw socket with UDP protocol
if ((send = socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP)) < 0)
{
printf("Could not process socket() [send].\n");
return EXIT_FAILURE;
}
// ICMP Socket
if ((recv = socket(AF_INET, SOCK_RAW, IPPROTO_ICMP)) < 0)
{
printf("Could not process socket() [recv].\n");
return EXIT_FAILURE;
}
dst.sin_family = AF_INET;
dst.sin_addr.s_addr = inet_addr("74.125.39.106");
dst.sin_port = htons(60001);
// Define time to life
if(setsockopt(send, IPPROTO_IP, IP_TTL, &ttl, sizeof(ttl)) < 0)
{
printf("Could not process setsockopt().\n");
return EXIT_FAILURE;
}
if(sendto(send, buffer, sizeof(buffer), 0, (struct sockaddr *) &dst, sizeof(dst)) < 0)
{
printf("Could not process sendto().\n");
return EXIT_FAILURE;
}
if((resp = recvmsg(recv, (struct msghdr *) &icmp, IP_RECVERR|MSG_ERRQUEUE)) < 0 )
{
printf("Could not process recvmsg().\n");
return EXIT_FAILURE;
}
close(send);
close(recv);
return 0;
}
I keep receiving "Could not process recvmsg()." and I have no clue anymore what else to try. I'd like to receive an ICMP answer and read its senders IP-Address.
Looking forward to helpfull hints.
Best regards!
I normally use
recvfrom(serversock, buf, 100, 0, (struct sockaddr *)&rcv,&size);
printf("Received packet from %s:%d\nData: %s\n\n", inet_ntoa(rcv.sin_addr), ntohs(rcv.sin_port), buf);